The present invention relates to semiconductor device manufacturing and, more particularly, to the removal or stripping of an ion implanted photoresist material and related photoresist material residue from a surface of a semiconductor structure during processing thereof.
The fabrication of integrated circuits (ICs) on a semiconductor substrate typically includes the use of a front-end-of-the line (FEOL) process which forms one or more semiconductor devices such as, for example, transistors, on a surface of the semiconductor substrate. In a typical FEOL process, various selected areas of the semiconductor substrate are exposed to ion implantation of impurities (e.g., dopants) such as, for example, phosphorus, boron or arsenic, in order to create p-type and/or n-type regions. The doping of the selected areas of the substrate begins with the deposition of a photoresist layer. The photoresist layer is typically dried and cured after deposition. The photoresist is photoactive, and can be modified by exposure to selected forms of radiant energy. Exposure of the photoresist is performed in a photolithography step where the substrate is exposed to radiant energy of selected wavelengths through a mask. This mask defines those areas of the photoresist-coated substrate, which are subjected to the radiation and those that are not. Typically, the areas of photoresist that are subject to the radiation are modified and can be removed by developing. This method of pattern transfer (from mask to substrate) leaves photoresist covering those areas of the substrate that were shielded by the mask.
Ion implantation is then employed to drive the impurity dopants into those areas of the substrate that are not protected by the photoresist. Subsequent to this step, all the photoresist must be removed before the substrates are annealed, oxidized or processed in diffusion furnaces. Currently, post-implant photoresist removal is performed by wet etching, dry etching or a combination of wet etching and dry etching. Wet etching processes typically involve the employment of a mixture of sulfuric acid and hydrogen peroxide to remove the resist and other organic contaminants that might be present. The photoresist can also be removed using dry etching processes, typically involving the use of plasma oxidation. In a plasma process, a source of oxidant, such as oxygen, is introduced into a plasma field excited either by radio frequency or microwave frequency energy.
The recent process trends in the manufacture of ICs have caused an increase in the level of doping. This has been achieved by increasing the energy and density of the ion flux directed at the substrate during the ion implantation process. As a consequence, the surface of the photoresist that shields certain areas of the substrate from the ion implantation process is itself modified. Due to the high energy and flux density, surface layers of the photoresist undergo chemical and physical modification. The higher temperatures resulting from the ion bombardment cause further baking and hardening of the top surface layer of the photoresist. Also, the ion flux causes implantation of the resist with the dopant atoms. Moreover, the photoresist undergoes significant cross-linking and becomes more resistant to post-implant removal processes. The modified surface layer of the ion implanted photoresist is typically referred to in the art as a crusted surface layer.
The conventional techniques of removing the ion implanted photoresist having the crusted surface layer also involve a combination of dry and wet etching, or a wet etch using sulfuric-acid-based chemistries, typically a mixture of sulfuric acid and hydrogen peroxide. A common drawback of all prior art strip methodologies includes the incomplete removal of the crusted photoresist present on the substrate surface post ion implantation.
Moreover, resist removal via conventional techniques has been shown to process excessive semiconductor material, e.g., Si, from the structure as well as dopant loss and possible damage to fragile semiconductor structures. Also, when metal gates are present on the surface of the substrate during the ion implantation process, the prior art resist stripping methods mentioned above would damage the metal gate by oxidizing the same.